## Wednesday, March 16, 2011

### Chains of Reasoning: Static Electricity #1

In the past, I've been guilty of blowing through the whole charge transfer/induced charge/polarization thing in a day, and not coming back to it well until the test.  That's sort of how we're trained, though, isn't it?  If there aren't any numbers, then it's "easy."

Of course, it's not true, and spending some time thinking about the concepts just on their own merits is vitally important to make sure all of those numbers go together well when you get there (assuming that you're writing good problems in the first place, that kids can't just see through and that actually require understanding!).  I'm hoping for some much better RC circuit understanding later. *crosses fingers*

So we started today, talking about protons, electrons, Ben Franklin being a jerk, polarization, induced charge, etc.  The big addition is some time for them to construct a reasoning chain about what happens to a negatively charged electroscope when you bring a positively charged object nearby, and the same for a negatively charged object.

The electroscope itself is awesome: there's conduction, repulsion, the opportunity to talk about polarization (bring a rubbed balloon near it, but don't actually transfer any charge) and induced charge (same, but touch the leaves with your finger briefly, then remove the balloon).

Here, though, we charged it with the balloon and thought about what'd happen when you bring a negative charge nearby:

...and a positive charge nearby:

These are the first explicit reasoning chains that we've done with honors physics, so we'll see how it goes tomorrow, when we go after Volta's Hail storm.

We did, BTW, make the simplifying assumption for our drawings that all of the charges are mobile (protons and electrons alike).  No, it's not true, but it's the assumption that we make for circuits anyway, and we do talk about it a bit.  There's an animation that does a good job of showing what happens when you keep track of all of the charges and don't let the protons move, though.  This would be a bear to draw, and I'm always worried about losing the physics in the laborious process of drawing, writing, building, etc.  Let's keep it simple for now.

Of course, there were the obligatory hair vs. Van de Graaff pictures:

...and Hugh's really bad at picking things up (image links to video):